Integrated circuit chip packages are typically formed by mounting an integrated circuit chip on a lead frame and coupling these two elements to form a package. The package may be encapsulated by, for example, injection molding the empty spaces of the internal area of the lead frame.
Different techniques have been used to prevent the mold material from leaving the internal area of a lead frame and moving to the external area during the encapsulation process. One approach has been the use of metal dambars. A lead frame may be a stamped metal structure including individual leads having internal, intermediate and external portions. The metal dambars constitute portions of the lead frame structure which join the intermediate portions of the individual leads.
According to a typical encapsulation process, a mold cavity is formed by sandwiching the lead frame between plates. The metal dambars provide a boundary to prevent the mold material from traveling to the external area of the lead frame and among the external portions of the leads. After the encapsulation process, the metal dambars are removed to electrically isolate the leads from one another. This may be accomplished by a precision stamping tool which mechanically cuts the dambars from between each individual lead. This is a time consuming and expensive process. Also, over time, lead frames are being made smaller and smaller and pin count is increasing. The lead frames are incorporating greater numbers of more narrow leads and the distance between leads is being reduced. Thus, greater precision is required to remove the dambars.
An alternative to using dambars is to fill the spaces between leads with an organic tape or paste. An organic material in liquid form may be dispensed along the intermediate portions of the leads to fill the spaces between the leads. The material may then be hardened by polymerization. This may be accomplished, for example, by ultraviolet radiation. One of the drawbacks to this process is that the application of the material is tedious and time consuming, and uniformity in application is virtually impossible. After the encapsulation process, the organic material may be left in place because it is a dielectric. However, typical materials used in this approach are generally not compatible with the service environment of the electronic device incorporating the package. For example, temperature cycling, vibration, exposure to gases, condensation and the like can cause the material to deteriorate. This can result in leakage and poor electrical isolation among the leads. Therefore, it is often desirable to remove the material. As with metal dambars, the removal process is expensive and time consuming, and requires precision instruments.
Another alternative to metal dambars is the application of a thermoplastic tape or a plastic preform to the lead frame. One or more sheets of tape or a plastic preform may be pressed against the leads while being heated. Heating causes the tape or preform to melt onto and between the leads. This approach has disadvantages similar to those described above in connection with using organic tape.
Another alternative to metal dambars is the use of two plastic film "linings" for the upper and lower halves of the mold cavity. The two films line the mold cavity and are drawn by a vacuum so that they are contoured to a surface of the cavity. At an intermediate temperature during the encapsulation process, the film flows into the spaces between leads to create a seal about the perimeter of the lead frame. After the encapsulation material is injected, the film may be stripped away and advanced to line and seal a mold cavity of a subsequent lead frame. This process is relatively costly, however, and requires special tools and equipment.
Other problems, shortcomings and disadvantages of known integrated circuit chip package encapsulation techniques and the elimination of metal dambars will be readily apparent to those having ordinary skill in the relevant art.